I know there was a thread a few weeks ago, but I can't find it.

I have been reading about interfaces, and I understand the concept, but my question is "So What?" I just don't see them being helpful other than to enforce standards. Should all of my classes have an interface? What does that gain me?

Also, when should I use an interface as opposed to inheritance? I am just not "getting" it and I figured it was about time to do something about it.